Defending Formula One constructors champions Red Bull have been forced to change their engine mapping for this weekend's Hungarian Grand Prix.

Motor racing's ruling body, the International Motoring Federation (FIA), on Thursday confirmed a clarification to the technical regulations that effectively closed a loophole in the rules.

The issue revolves around using exhaust gas for an aerodynamic advantage, as was allowed last season.

Red Bull are believed to have been running their cars - driven by defending drivers world champion German Sebastian Vettel and title contender Australian Mark Webber - with mapping that allowed them to run with reduced torque in the mid-rpm range.

This is perceived to have given them an illegal performance advantage that was identified last weekend at the German Grand Prix where the stewards, when presented with an FIA report on the matter, ruled that the cars were not breaking the rules as written.

The FIA has since re-written the rules and clarified the issues in order to ensure that the Red Bull cars will not continue to take any advantage.

The FIA has issued a clarification of Article 5.5.3 within F1's technical regulations to the teams ahead of this weekend's race.

It is understood that the teams will be required to supply one engine map - as a reference - that they used during the first four events of this season, which must then be approved by the governing body.

Once passed by the FIA, the engine torque curves above 6,000rpm must not vary by more than plus or minus two per cent from that reference map.

Teams will be allowed to make specific requests for changes when races take place in 'exceptional atmospheric conditions'.

Red Bull have yet to make any official reaction to the new ruling, but McLaren managing director Jonathan Neale said he could not quantify the impact the clarification would have on Red Bull's performance.

"The honest answer is I really don't know," Neale said.

"That was quite an unusual step (in Germany) - I don't think the FIA would have referred to the stewards unless they had very serious concerns. It's really not for us to know or tell exactly what the Renault engine is doing in the Red Bull and therefore how much advantage they gain.

"But I know we are not the only ones on the grid who are looking at it very carefully."

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